This is a work of Gothic fiction that does not really live up to its hype. It has some good features, notably the setting in and below the Paris Opera House, but it is silly without being charmingly silly.
Its very much a melodramatic potboiler. It has a number of interesting ideas, but in the end the ideas are not well executed. Its told as if the author had pieced it together from facts, but that is not very well done. In particular, the character of the Phantom is wasted. If you compare that character to say Frankenstein's Monster, Dracula or Mr. Hyde there is not much really there.
The trickery and mystery is all unreal in a way that does not lead to suspense or raw enjoyment. Its a bunch of interesting settings and characters that are wasted. There is also a boring approach to telling the story. With a better writer a lot could have been done with the story and it could have been taken in a number of interesting directions. It is really an early twentieth century mass market horror/adventure tale, that has lived because of ingenious ideas that can be adapted well into other mediums, but is really sort of dull when you read it. Fors some reason, if a book is French and old people think it is literature no matter how trashy it is (e.g. The Count of Monte Cristo). This is very much just a trashy not well written book with some good ideas for visuals (albeit these visuals are not realized well in the book).